In the process for forming images utilizing the electronic photography, a photosensitive member is uniformly charged with static electricity, an electrostatic latent image is formed by exposing the uniformly charged member to light so that the electrostatic charge in the exposed portions is dissipated, the latent image is made visible by development by attaching a toner to the electrostatic image, the visualized image is transcribed to a material such as paper, and the transcribed image is fixed by a means for fixing such as heating. The process for the development includes the single component process using a single type of a magnetic or non-magnetic toner and the double component process using two types of powder materials that are a toner and a carrier. The apparatus can be made smaller and simpler in the development in accordance with the single component process. In particular, the development in accordance with the non-magnetic single component process has a characteristic in that color toners providing bright images can be used.
In an apparatus for forming images by development in accordance with the non-magnetic single component process, hydrophobic silica is frequently added to the mother toner particles so that the amount of the electrostatic charge of the toner on the development roller is kept uniform during printing for a long period of time. When silica alone is added externally, background becomes high and the electrostatic charge tends to be unstable under fluctuations in the environment. To overcome these problems, titanium oxide, barium titanate, strontium titanate or magnetite is added externally.
For example, as the negatively charged toner which exhibits excellent stability of the electrostatic charge during repeated uses and under fluctuations in the environment, exhibits excellent property for transcription to paper and reproducibility of black color and can overcome the problem of filming to the photosensitive member and blurred dots during fusing, a negatively charged toner which contains a polyester-based resin as the binder resin and a boron-based chelate compound as the charge control agent and to which hydrophobic silica and a metal titanate are added from the outside in amounts such that the ratio of the amounts by weight of hydrophobic silica to the metal titanate is 5:1 to 1:1.2, is proposed. It is described that the decrease in the amount of electrostatic charge under an environment of a high temperature and a high humidity can be prevented and the stability of electrostatic charge under fluctuations in the environment can be improved by using hydrophobic silica and a metal titanate (Patent Reference 1). However, when hydrophobic silica and a metal titanate are added externally to the mother particles of a toner, aggregates of hydrophobic silica and the metal titanate tend to be formed. The formed aggregates tend to be separated from the toner particles to contaminate members of the printer such as the charge roller and the development roller, and print defects such as background and starvation of filled images tend to arise.
As the non-magnetic single component toner which can provide excellent density of formed images and suppress staining of the background simultaneously, a non-magnetic single component toner which contains 0.2 to 5 parts by weight of barium titanate formed in accordance with the liquid phase process and having a BET specific surface area of 0.5 to 5.0 m2/g per 100 parts by weight of the toner, is proposed (Patent Reference 2). However, since barium titanate having a BET specific surface area of 0.5 to 5.0 m2/g has a great particle diameter, barium titanate tends to be separated from the toner particles during repeated printing for a long period of time. Therefore, reproducibility of dots and narrow lines decreases, and the consumption of the toner increases.
Many of the recent apparatuses for forming full color images have a member for intermediate transfer, and transferability of the toner is important. Toner particles having a spherical shape is more advantageous from the standpoint of improving the transfer rate than toner particles having a sharp shape such as toner particles obtained by mixing and pulverization since the toner particles having a spherical shape have smaller areas of contact with the photosensitive member and the member for the intermediate transfer and exhibit smaller force of adhesion.
The toner particles having a spherical shape has another advantage in that the particles can be more uniformly charged. In the development using a non-magnetic single component toner, a thin layer is more easily formed on the development roller and the electrostatic charge is more easily stabilized when the toner particles having a spherical shape are used.
As the toner particles having a spherical shape, toner particles prepared in accordance with the suspension polymerization process or the emulsion polymerization process with aggregation and toner particles prepared by rounding toner particles obtained in accordance with a conventional process of mixing and pulverization by a heat treatment, are known. The toners prepared in accordance with the polymerization process have problems in that agents used in the polymerization such as surfactants are left remaining on the surface of the toner and adversely affect the property for electrostatic charge of the toner and that a very great amount of the initial investment in the apparatus is required. The toner prepared in accordance with the suspension polymerization process has a further problem in that, when the toner left remaining on the photosensitive member after the transcription is cleaned with an elastic blade, incomplete cleaning tends to take place since the shape of the particles is almost perfectly spherical,
Examples of the toner formed by rounding by the heat treatment include a toner described in Patent Reference 3 [Japanese Patent Application Laid-Open No. Heisei 11 (1999)-295929]. However, the agents added from the outside tend to be separated since the surface of the toner particles is smooth and wax is present on the surface in a significant amount. It occasionally takes place that the wax component works as the binder, and the agents added from the outside contaminate the development roller and the charge roller.    [Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 11 (1999)-133669    [Patent Reference 2] Japanese Patent Application Laid-Open No. 2002-107999    [Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 11 (1999)-295929